Derivative Instruments - Summary of Average Monthly Notional Value of Future Contracts Outstanding (Detail) - USD ($)	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Weighted Average [Member] \| Commodity Futures Contract [Member]
Derivative [Line Items]
Average Notional Value	$ 690,035,072	$ 760,714,902

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

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